Thanks for your response -- I have been able to verify the correct operation. The main problem is that the knowledgebase articles refer to either Labview or the no longer active NiDAQmx-Base, and MI-MAX does not support setting properties at all. That leaves us LabWindows users out in the cold. But enough complaining about NI and their inadequate documentation,
the function that works is:
DAQmxSetDOOutputDriveType
This is documented in the "NI-DAQmx C Reference Help", and not in the LabWindows NiDAQmx library help.